Long horizontal well completions have become more viable for producing hydrocarbons, especially in deepwater reservoirs. Gravel packing with screens has been used to provide sand control in horizontal completions. A successful, complete gravel pack in the wellbore annulus surrounding the screen, as well as in the perforation tunnels if applicable, can control production of formation sand and fines and prolong the productive life of the well.
Cased-hole gravel packing requires that the perforations or fractures extending past any near-wellbore damage as well as the annular area between the outside diameter (OD) of the screen and the inside diameter (ID) of the casing be tightly packed with gravel. See Brochure: “Sand Control Applications,” by Halliburton Energy Services Inc., which is incorporated herein by reference for all purposes. The open-hole gravel-pack completion process requires only that the gravel be tightly packed in the annulus between the OD of the screen and the openhole.
Several techniques to improve external gravel-pack placement, either with or without fracture stimulation, have been devised. These improved techniques can be performed either with the gravel-pack screen and other downhole equipment in place or before the screen is placed across the perforations. The preferred packing methods are either 1) prepacking or 2) placing the external pack with screens in place, combined with some sort of stimulation (acid-prepack), or with fracturing or acidizing. The “acid-prepack” method is a combination stimulation and sand control procedure for external gravel-pack placement (packing the perforations with gravel). Alternating stages of acid and gravel slurry are pumped during the treatment. The perforations are cleaned and then “prepacked” with pack sand.
Combination methods combine technologies of both chemical consolidation and mechanical sand-control. Sand control by chemical consolidation involves the process of injecting chemicals into the naturally unconsolidated formation to provide grain-to-grain cementation. Sand control by resin-coated gravel involves placing a resin-coated gravel in the perforation tunnels. Resin-coated gravel is typically pumped as a gel/gravel slurry. Once the resin-coated gravel is in place, the resin sets up to form a consolidated gravel filter, thereby removing the need for a screen to hold the gravel in place. The proppant pumped in a frac treatment may be consolidated into a solid (but permeable) mass to prevent proppant-flow back without a mechanical screen and to prevent formation sand production. U.S. Pat. No. 5,775,425, which is incorporated herein by reference for all purposes, discloses an improved method for controlling fine particulates produced during a stimulation treatment, including the steps of providing a fluid suspension including a mixture of a particulate coated with a tackifying compound and pumping the suspension into a formation and depositing the mixture within the formation.
A combined fracturing and gravel-packing operation involves pumping gravel or proppant into the perforations at rates and pressures that exceed the parting pressure of the formation. The fracture provides stimulation and enhances the effectiveness of the gravel-pack operation in eliminating sand production. The fracturing operation produces some “restressing” of the formation, which tends to reduce sanding tendencies. See Brochure: “STIMPAC Service Brochure,” by Schlumberger Limited, which is incorporated herein by reference for all purposes. The high pressures used during fracturing ensure leakoff into all perforations, including those not connected to the fracture, packing them thoroughly. Fracturing and gravel packing can be combined as a single operation while a screen is in the well.
“Fracpacking” (also referred to as “HPF,” for high-permeability fracturing) uses the tip-screenout (TSO) design, which creates a wide, very high sand concentration propped fracture at the wellbore. See M. Economides, L. Watters & S. Dunn-Norman, Petroleum Well Construction, at 537–42 (1998), which is incorporated herein by reference for all purposes. The TSO occurs when sufficient proppant has concentrated at the leading edge of the fracture to prevent further fracture extension. Once fracture growth has been arrested (assuming the pump rate is larger than the rate of leakoff to the formation), continued pumping will inflate the fracture (increase fracture width). The result is short but exceptionally wide fractures. The fracpack can be performed either with a screen and gravel-pack packer in place or in open casing using a squeeze packer. Synthetic proppants are frequently used for fracpacks since they are more resistant to crushing and have higher permeability under high confining stress.
In a typical gravel pack completion, a screen is placed in the wellbore and positioned within the zone which is to be completed. The screen is typically connected to a tool which includes a production packer and a cross-over port, and the tool is in turn connected to a work string or production string. A particulate material which is usually graded sand, often referred to in the art as gravel, is pumped in a slurry down the work or production string and through the cross-over port whereby it flows into the annulus between the screen and the wellbore and into the perforations, if applicable. The liquid forming the slurry leaks off into the subterranean zone and/or through the screen which is sized to prevent the sand in the slurry from flowing therethrough. As a result, the sand is deposited in the annulus around the screen whereby it forms a gravel pack. The size of the sand in the gravel pack is selected such that it prevents formation fines and sand from flowing into the wellbore with produced fluids.
The “Alpha-Beta” gravel-pack technique has been used to place a gravel pack in a horizontal hole. See Dickinson, W. et al.: “A Second-Generation Horizontal Drilling System,” paper 14804 presented at the 1986 IADC/SPE Drilling Conference held in Dallas, Tex., February 10–12; Dickinson, W. et al.: “Gravel Packing of Horizontal Wells,” paper 16931 presented at the 1987 SPE Annual Technical Conference and Exhibition held in Dallas, Tex., September 27–39; and M. Economides, L. Watters & S. Dunn-Norman, Petroleum Well Construction Section 18–9.3, at 533–34 (1998), which are all incorporated herein by reference for all purposes.
The Alpha-Beta method primarily uses a brine carrier fluid that contains low concentrations of gravel. A relatively high flow rate is used to transport gravel through the workstring and cross-over tool. After exiting the cross-over tool, the brine-gravel slurry enters the relatively large wellbore/screen annulus, and the gravel settles on the bottom of the horizontal wellbore, forming a dune. As the height of the settled bed increases, the cross-sectional flow area is reduced, increasing the velocity across the top of the dune. The velocity continues to increase as the bed height grows until the minimum velocity needed to transport gravel across the top of the dune is attained. At this point, no additional gravel is deposited and the bed height is said to be at equilibrium. This equilibrium bed height will be maintained as long as slurry injection rate and slurry properties remain unchanged. Changes in surface injection rate, slurry concentration, brine density, or brine viscosity will establish a new equilibrium height. Incoming gravel is transported across the top of the equilibrium bed, eventually reaching the region of reduced velocity at the leading edge of the advancing dune. In this manner, the deposition process continues to form an equilibrium bed that advances as a wave front (Alpha wave) along the wellbore in the direction of the toe. When the Alpha wave reaches the end of the washpipe, it ceases to grow, and gravel being transported along the completion begins to back-fill the area above the equilibrium bed. As this process continues, a new wave front (Beta wave) returns to the heel of the completion. During deposition of the Beta wave, dehydration of the pack occurs mainly through fluid loss to the screen/washpipe annulus.
Successful application of the Alpha-Beta packing technique depends on a relatively constant wellbore diameter, flow rate, gravel concentration, fluid properties and low fluid-loss rates. Fluid loss can reduce local fluid velocity and increase gravel concentration. Both will increase the equilibrium height of the settled bed or dune. Additionally, fluid loss can occur to the formation and/or to the screen/washpipe annulus.
The key to successful frac packs and gravel packs is the quantity of gravel placed in the fracture, perforations and casing/screen annulus. The development of bridges in long perforated intervals or highly deviated wells can end the treatment prematurely, resulting in reduced production from unpacked perforations, voids in the annular gravel pack, and/or reduced fracture width and conductivity.
U.S. Pat. No. 5,934,376, which is incorporated herein by reference for all purposes, discloses a sand control method called CAPS™, for concentric annular packing system, developed by Halliburton Energy Services, Inc. See also Lafontaine, L. et al.: “New Concentric Annular Packing System Limits Bridging in Horizontal Gravel Packs,” paper 56778 presented at the 1999 SPE Annual Technical Conference and Exhibition held in Houston, Tex., October 3–6, which is incorporated herein by reference for all purposes. CAPS™ basically comprises the steps of placing a slotted liner or perforated shroud with an internal sand screen disposed therein, in the zone to be completed, isolating the perforated shroud and the wellbore in the zone and injecting particulate material into the annuli between the sand screen and the perforated shroud and the wellbore to thereby form packs of particulate material therein. The system enables the fluid and sand to bypass any bridges that may form by providing multiple flow paths via the perforated shroud/screen annulus.
The CAPS™ assembly consists of a screen and washpipe, with the addition of an external perforated shroud. The CAPS™ concept provides a secondary flow path between the wellbore and the screen, which allows the gravel slurry to bypass problem areas such as bridges that may have formed as the result of excessive fluid loss or hole geometry changes.
Flow is split among the three annuli. A gravel slurry is transported in the outer two annuli (wellbore/shroud and shroud/screen), and filtered, sand-free fluid is transported in the inner annulus (screen basepipe/washpipe). If either the wellbore/shroud or shroud/screen annulus bridges off, the flow will be reapportioned among the annuli remaining open.
One problem area in horizontal gravel packs is the ability to bypass problems zones such as shale streaks. Horizontal completions often contain shale zones, which can be a source of fluid loss and/or enlarged hole diameters with subsequent potential problems during the gravel pack completion. In addition, shale zones may complicate selection of the appropriate wire-wrapped screen gauge. Another potential problem of shale zones is sloughing and hole collapse after the screen is placed. In open hole wellbores sloughing of shale or unstable formation materials can cause premature screen out during gravel pack treatment, leaving most of the well bore annulus unpacked or voided.
Completion of horizontal wells as open holes leaves operators with little or no opportunity to perform diagnostic or remedial work. Many horizontal wells that have been producing for several years are now experiencing production problems that can be attributed to the lack of completion control. The main reason for alternative well completions is that open holes do not allow flexibility for zonal isolation and future well management. The competence of the formation rock is a first consideration in deciding how to complete a horizontal well. In an unconsolidated formation, sand production often becomes a problem.
One completion design for horizontal wells includes the use of slotted or blank liner, or sand-control screen, separated by external-casing packers (ECP's). Generally, the packers are hydraulically set against the formation wall. However, gravel packing operations would be impossible because the ECP's become barriers, blocking the flow paths of gravel slurry. Gravel placement in the zones below the isolated zone is prevented.
Thus, there are needs for improved methods and apparatus for completing wells, especially in the case of open-hole well bores where sloughing problems may occur or to allow flexibility for zonal isolation and well management.